Liquid crystal does not emit any light by itself. Therefore, an external light source is required in a liquid crystal display (LCD) module. Typically, a backlight module is used in a transmission liquid crystal display as a light source. Cold cathode fluorescent lamps (CCFL) or light emitting diodes are the light source in a backlight module. High brightness efficiency and long lifetime make the cold cathode fluorescent lamps popular for use in backlight modules.
Light modules usually can be categorized into back type and front type, wherein the back type further includes edge-light light module and direct-light module. The edge-light backlight module can provide an average brightness with an efficiency of about 50%. Moreover, the edge-light backlight module is thin and lightweight. Therefore, it can be used in a portable color LCD. In comparison, the brightness efficiency of the direct-light backlight module is about 60%. However, it is heavier and thicker. Therefore, the direct-light backlight module is always used in LCD monitors and LCD televisions.
Furthermore, when the LCD monitor is larger than 17 inches, the weight of the direct-light backlight module is actually less than the weight of the edge-light backlight module. Therefore, there is the trend to use the direct-light backlight module for large-scale LCD televisions.
Referring to FIG. 1A, a schematic top view of a conventional direct-light backlight module is illustrated. Several cold cathode fluorescent lamps 100 are arranged in parallel to each other so as to provide light to the LCD panel (not shown in this figure). FIG. 1B illustrates a schematic sectional view along the AA′ line of FIG. 1A. The light emitted from the CCFLs 100 is reflected by the reflective plate 102. The reflected light is then diffused through the diffuser plate 104 to provide a uniform backlight to the LCD panel. However, the image displayed on the LCD panel is often degraded with what appear to be shadows caused by the presence of the CCFLs arranged underneath the panel. A backlight module that does not cause the shadows on the display is thus required.